From the linked ticket:
There is an easy fix, copy the file after which both vm-INBOX.ulaw and vm-INBOXs.ulaw exist.
Get a list of all the relevant files:
ll /var/lib/asterisk/sounds/nl/vm-INBOX*
make copies with the command:
cp /var/lib/asterisk/sounds/nl/vm-INBOX.ulaw /var/lib/asterisk/sounds/nl/vm-INBOXs.ulaw
when all done fix file ownership/perms with:
fwconsole chown